President Reagan, who last week said he had resolved his differences at the summit with Soviet leader Mikhail Gorbachev over strategic defense programs, acknowledged yesterday "we are in disagreement" over the issue and expressed hope that it would not be a stumbling block to reducing strategic offensive arms.

In a brief picture-taking session, Reagan was asked about Gorbachev's nationally broadcast assertion to the Soviet people Monday that the Washington summit disagreement over strategic defenses could derail the "nascent" process of nuclear disarmament.

In their joint statement concluding the summit, the two leaders essentially agreed to disagree over strategic defense, allowing each nation to research, develop, and test such systems "as required."

On Friday, Reagan said that this formula "resolves" the issue.

On Saturday, in his weekly radio address, the president said an agreement reducing strategic offensive arms "could be another historic achievement -- provided the Soviets don't try to hold it hostage to restrictions on SDI," the space-based Strategic Defense Initiative.

But Gorbachev in his address later cautioned that "the nascent turn" in disarmament could be "undermined" by "dangerous tendencies" of those who say there are no superpower differences on space defense tests.

Yesterday, Reagan conceded that the two leaders remain in disagreement and he said, "It was a simple thing. He took his position, we took ours and it was put that way in the joint communique. We are going forward."

Asked if that meant the leaders agreed to disagree, Reagan said, "Yes."

Asked whether the disagreement could again become an obstacle to a strategic arms agreement, Reagan said simply, "No."

But Paul Nitze, a senior arms control adviser, expressed a contrary view during a luncheon address to the National Press Club here.

"I fully expect that . . . this will be raised," Nitze said of possible Gorbachev objections to SDI as a condition for signing a strategic arms accord during Geneva negotiations or a summit meeting in Moscow next year.

White House spokesman Marlin Fitzwater said the White House had "no assurances" from Gorbachev that he would not link negotiations on the missile defense issue with those on cutting strategic offensive arms. Reagan has expressed hope for agreement next year on a treaty to reduce offensive arsenals by half.

Nitze also said the two sides disagreed on the meaning of the Reagan-Gorbachev pledge last week to observe the ABM Treaty "as signed" in 1972. The Soviets say it constrains realistic SDI tests in space; the United States argues it does not.

Nitze said "it will be very difficult, but not impossible" to complete negotiations on a strategic arms treaty before the end of Reagan's term. Despite what he called the summit's "significant headway" on strategic arms, Nitze said disputes remain.